1. Field of the Invention
The present invention relates to a display device including a self light emitting element, and a method of driving the display device.
2. Description of the Related Art
In recent years, organic EL displays have been actively developed, and have achieved a significant advancement. In a display device including a self light emitting element such as an organic EL element, light emission may be controlled pixel by pixel, and hence there are advantages in contrast and viewing angle characteristics. When used in video display or the like, there is also a merit that reduction in power consumption may be achieved because an average display gradation is low. Meanwhile, when the characteristics of the light emitting element themselves are deteriorated by its use, luminance reduction occurs in accordance with a usage history of each pixel. The luminance reduction occurs at a predetermined pattern depending on displayed images or the usage, and in some cases, the luminance reduction may be visually recognized as “screen burn-in.”
In a case where an organic EL element is used as the light emitting element, light emission intensity is proportional to a current flowing through the element. The ratio between the light emission intensity and the current flowing through the element is called a current luminous efficiency. Normally, the current luminous efficiency is determined based on an organic material forming the light emitting element, an element structure, an interface state, or the like, and the current luminous efficiency is uniform across the entire display region. Therefore, when uniform display characteristics are desired to be obtained, it is only necessary to control, pixel by pixel, the current to be supplied to the light emitting element, so as to obtain uniform display. In an active matrix type organic EL display, the current is controlled by a thin film transistor (TFT) element provided in each pixel, and thus the organic EL element is driven. Generally, a low-temperature polycrystalline silicon TFT or the like is used as the TFT element.
As characteristics of the low-temperature polycrystalline silicon TFT, there is a problem in that, because of grain boundary scattering of conduction electrons, fluctuation in mobility or in turn-on voltage occurs among the pixels. Therefore, efforts have been made to obtain uniform display characteristics by suppressing the fluctuation in mobility or in turn-on voltage and by correcting the fluctuation, to thereby enable uniform pixel current supply. For example, Japanese Patent Application Laid-open No. 2005-217214 describes a technology in which the crystal-growth direction of the polysilicon is controlled to obtain crystalline grains of uniform shapes. Further, there have been proposed many technologies for suppressing fluctuation in display characteristics caused by fluctuation in threshold voltage of the TFT, by adding, to a pixel circuit, a function to offset a threshold voltage of a drive TFT. For example, Japanese Patent Application Laid-open No. 2008-203387 is proposed.
Here, the conventional technologies described above are based on the presupposition that the organic EL element maintains the in-plane uniformity of the current luminous efficiency. However, in actual use, the organic EL element itself is deteriorated by its use, and the current luminous efficiency is accordingly reduced. Reflecting the difference of the usage history among the pixels, the current luminous efficiency is reduced in different speed among the pixels. Depending on the usage of the display device and the displayed images, the difference of the deterioration speed among the organic EL elements may be increased to an extent not negligible. In this case, the difference is visually recognized as display luminance unevenness and screen burn-in. Generally, an organic EL display device life is defined by a luminance half-life. The luminance unevenness and the screen burn-in reach allowable limits thereof with the luminance difference of several percent, and hence the luminous efficiency reduction of the organic EL element is a cause of a significant reduction in device life. Therefore, there is a demand to compensate for the display luminance reduction caused by the current luminous efficiency reduction of the organic EL element.